Alzheimer's disease is the leading cause of dementia in the elderly. The brains of patients with Alzheimer's disease contain characteristic structural lesions known as plaques and tangles. Plaques will not be considered here. Neurofibrillary tangles represent intracellular accumulations of paired helical filaments (PHFs), which are quite unlike any of the normal structural elements of the neuronal cytoskeleton. Tangles become extracellular on the death of the affected cell. PHFs are also found in abnormal neurites associated with neuritic plaques and as an extensive distribution of fine neurophil threads throughout affected regions of the brain. The extent of neurofibrillary degeneration found post mortem appears to provide the most reliable pathological correlate of the degree of dementia observed in life. These topics are discussed further in references 1 and 2.
PHFs are made of microtubule-associated protein tau in a hyperphosphorylated state (3-8). Hyperphosphorylation of tau is known to result in its inability to bind to microtubules (9, 10) and is believed to precede PHF assembly (11). However, it is unclear whether hyperphosphorylation of tau is either necessary or sufficient for PHF formation. A major reason for this lack of understanding is that it has not hitherto been possible to form paired helical-like filaments for full-length tau protein either in vitro or in vivo.
Tau protein consists in adult human brain of six isoforms, produced from a single gene by alternative mRNA splicing. The isoforms, which range in size from 352 to 441 amino acids, contain towards the carboxyl terminus a tandem repeat region with three or four homologous stretches of 31 or 32 amino acids. The other isoforms are generated by the presence of a 29- or 58- amino-acid insertion in the amino-terminal half of the protein.
The repeat region of tau represents the microtubule-binding domain, and the amino-terminal half of the protein appears to form an arm-like projection from the surface of the microtubule.
When tau protein is in hyperphosphorylated state (PHF-tau) the protein is unable to bind to microtubules and assembles into PHFs or structural variants known as straight filaments (SFs). The repeat region of tau constitutes the core of the resulting filaments.
See references 31 and 32 for further discussion of tau protein.
The present invention is based on the discovery that suitable sulphated carbohydrates can induce in vitro assembly of tau protein into filaments like those in Alzheimer's disease under physiological conditions in an essentially quantitative manner. This discovery forms the basis of an in vitro assay or screen for agents that inhibit the assembly and so may have potential use in the treatment of Alzheimer's disease.